1. Field of the Invention
The present invention relates to a system and method for excavating a wellbore. More specifically, the invention relates to a system and method for continuously rotating a drill string in the wellbore while lengthening the drill string.
2. Description of the Related Art
Hydrocarbon producing wellbores extend subsurface and intersect subterranean formations where hydrocarbons are trapped. The wellbores generally are created by drill bits that are on the end of a drill string, where a drive system above the opening to the wellbore rotates the drill string and bit. Cutting elements are usually provided on the drill bit that scrape the bottom of the wellbore as the bit is rotated and excavate material thereby deepening the wellbore. Drilling fluid is typically pumped down the drill string and directed from the drill bit into the wellbore. The drilling fluid flows back up the wellbore in an annulus between the drill string and walls of the wellbore. Cuttings produced while excavating are carried up the wellbore with the circulating drilling fluid.
Drill strings are typically made up of tubular sections attached by engaging threads on ends of adjacent sections to form threaded connections. New tubular sections are attached to the upper end of the drill string as the wellbore deepens and receives more of the drill string therein. In a conventional rig operation, rotation of the drill string is temporarily suspended each time a tubular section is added to the drill string. When the drill string is not rotating, there is a risk that a portion of the drill string can adhere to a sidewall of the wellbore.